1. Field of the Invention
The present invention relates to a digital watermark processing technique applied to embed watermark information into digital data, such as image data and sound data.
2. Description of the Related Art
Information has been digitized with advancement of computer networks like the Internet and enables many users to readily access required information. Such technological innovation, however, causes copyrighted digital contents to be readily duplicated without any permission of the copyright holders. The problem of illegal copies and resulting copyright infringement has been significantly noticeable. With a view to preventing copyright infringement of sounds and images, which are primary information of the digital contents, digital watermark techniques have been developed to embed watermark information, such as copyright information, into sound data and image data.
One of the digital watermark techniques carries out frequency transformation (orthogonal transformation) of image data and sound data and embeds watermark information into resulting transform coefficients. The conventional digital watermark technique divides data into multiple blocks and applies frequency transformation onto each block. In the course of extraction of the embedded watermark information, this technique requires correction of the position and the shape of the data blocks. Especially when the data with the embedded watermark information is subjected to geometrical deformation, sophisticated matching process is required to correct the deviated position and the deformed shape of the data blocks due to the geometrical deformation.